Patients suffering from bronchoconstriction associated with reversible obstructive airways diseases are generally treated using a bronchodilator, to relax the bronchial smooth muscle.
Bronchodilators in use today generally fall into two classes, the β2-selective adrenoceptor agonists, such as albuterol (salbutamol), salmeterol and formoterol, and the muscarinic receptor antagonists, such as ipratropium and tiatropium.
β2-Selective adrenoceptor agonists may cause adverse effects, and these may in part be due to activation of the β1-adrenoceptor. The selectivity of an agonist for the β2-adrenoceptor receptor is therefore very important, because it limits the dose that can be given and so affects the magnitude of bronchodilations and the frequency of dosing.
A long duration of action is important to patients, not only to minimize the time spent taking the drug, but also to avoid having to take the drug during inconvenient times, for example at work, school or during the night. Some of the more recent β2-selective adrenoceptor agonists, in particular salmeterol and formoterol, have a long duration of action, typically about 12 hours. Formoterol has a particular advantage that it also has a fast onset of action. However, formoterol is extremely potent, which makes it very difficult to formulate, especially for administration using a metered dose inhaler in a manner that results in uniform drug delivery via aerosol dose after dose (i.e., dose content uniformity). Furthermore, it is unstable in aqueous solution, which means that solutions for administration using a nebuliser have to be kept refrigerated for a majority of their post-manufacture shelf life.
Formoterol is one of a group of α-aminomethylbenzyl alcohol derivatives for which patent applications were filed during the early nineteen seventies, for example GB 1 415 256. Perhaps because of the difficulties associated with formulating the compound, it took a long time to be commercialized. The compound contains two chiral centers, and hence is capable of existing and being isolated in four stereoisomeric forms. The compound was firstly commercialized as a racemic mixture of the active (R,R)- and inactive (S,S)-isomers, in a dry powder formulation, then more recently as the active (R,R)-isomer in a nebuliser solution. It is also known, for example from U.S. Pat. No. 6,303,145, that the (S,R) isomer of formoterol is active. However, like the (R,R)-isomer, this compound is unstable at ambient temperature in aqueous solution and hence nebuliser solutions would need to be stored refrigerated.